How Often the Demand Charge Misses a Business's Real Peak: Six ComEd Meters, Three Years

A ComEd commercial demand charge is set by the highest half hour of demand between 9 AM and 6 PM on a weekday. A building's actual highest half hour can land anywhere. The question this page answers with data rather than a shrug is how often those two are different, by how much, and for which kinds of business. We took six ComEd commercial accounts, three years of 30-minute meter data each, 214 billing months in all, and compared the physical peak of each month to the interval the tariff would have billed.

The rule, and how we reconstructed it

ComEd's Distribution Facilities Charge for these accounts bills the highest site-coincident demand, all meters at the site summed within the same 30 minute interval, that falls inside the on-peak window: 9:00 AM to 6:00 PM, Monday to Friday. We applied exactly that rule to each month of each export. For one of the six sites, the distillery, the reconstruction was checked against a real bill for September 2025 and matched the billed 40.91 kW and the $596.88 charge to the cent. For the other five, the billed figure on this page is the rule applied to the meter, not a bill line, and we have not priced it.

Sites are named by type only. Every number is a monthly aggregate; no interval and no account detail appears here.

What the six meters showed

business	months	physical peak outside the window	where it landed	typical gap when outside	largest gap
bowling alley	35	27 (77 percent)	19 weekday evenings, 8 weekends	12 percent	30 percent
weekend-peaking site	41	28 (68 percent)	25 weekends, 3 weekday mornings	9 percent	55 percent
seasonal retail site	34	9 (26 percent)	8 weekends, 1 weekday morning	5 percent	11 percent
distillery, two meters	35	4 (11 percent)	3 weekday mornings before 9, 1 evening	11 percent	64 percent
auto repair shop	34	1 (3 percent)	1 weekday morning	28 percent	28 percent
print shop, four meters	35	0	never	none	none

The gap is the physical peak as a percentage above the billed peak. A gap of 12 percent means the building's real highest half hour was 12 percent higher than the half hour that set the charge.

Three kinds of business, three answers

The evening business. The bowling alley's demand peaks when the lanes are full, and that is 7 to 9 PM. In 27 of 35 months its real peak fell outside the window, and in 19 of those it was a weekday evening after 6. The half hour ComEd billed was typically 12 percent below the building's true maximum, and once 30 percent below. Its billed peak usually landed at 5 PM, the last hour of the window, as the evening ramp began.

The weekend business. The weekend-peaking site's real peak fell on a Saturday or Sunday around noon in 25 of 41 months. Its weekday billed peak ran 9 percent below the weekend maximum in a typical month, and 55 percent below in the widest one. The seasonal retail site shows the same pattern more mildly: 8 weekend peaks in 34 months, at a 5 percent gap.

The weekday production business. The print shop, four meters summed, peaked inside the window in every one of 35 months, most often between 1 and 3 PM. The auto repair shop missed once in 34, on a morning start before 9. For these two, the billed peak and the real peak are the same number, and nothing about the window helps or hurts them.

The boundary case. The distillery is the odd one. It peaked inside the window in 31 of 35 months, and three of its four misses came together in the summer of 2025, when its morning ramp topped out at 7:30 or 8:30 AM, before the window opened, and the billed interval was the lower reading at 9:00. In July 2025 the physical peak was 64 percent above the billed one. The fourth miss, a June 2024 evening, was 8 percent. Whether that was luck or a schedule change is a question for the operator; the data only shows that the building's biggest half hour crossed the 9 AM line.

What this does and does not mean

It does not mean the evening and weekend businesses are saving money. It means the demand charge they pay describes a smaller building than the one they run. The bowling alley's wires carry a 7 PM load 12 percent above anything on its bill. That is worth knowing for two reasons. The first is that the delivery class and any equipment sizing are about the real peak, not the billed one. The second is that the window is a tariff rule, and tariff rules change; a business whose peak sits just outside the window is exposed to any redefinition of it in a way a weekday producer is not.

For the weekday production businesses it means something simpler: the interval that sets the charge is the one to find, because it is also the real one. And for a business near the boundary, like the distillery at 8:30 versus 9:00, a half hour of scheduling is the difference between a peak that is billed and one that is not, in either direction.

What none of these numbers tell you is what your own building does. The six sites span 3 percent to 77 percent on the same rule. The only way to place yours is to run the rule on your own meter, which is what the interval data analysis tool does from your ComEd export.